Make finding process id in timeout handler overridable

Currently JTreg skips timeout handler execution in cases process id isn't found. That makes it impossible to run timeout handler w/ pre JDK 9 as stable jdk on non-unix systems.

Moving PID determination to custom timeout handler logic will allow us to use custom logic to get pid, e.g. use win32 API to get PID from j.l.ProcessImpl on windows platforms.